Java如何自定义线程池中队列
背景
业务交互的过程中涉及到了很多关于SFTP下载的问题,因此在代码中定义了一些线程池,使用中发现了一些问题,
代码类似如下所示:
public class ExecutorTest {
private static ExecutorService es = new ThreadPoolExecutor(2,
100, 1000, TimeUnit.MILLISECONDS
, new ArrayBlockingQueue<>(10));
public static void main(String[] args) {
for (int i = 0; i < 10; i++) {
es.submit(new MyThread());
}
}
static class MyThread implements Runnable {
@Override
public void run() {
for (; ; ) {
System.out.println("Thread name=" + Thread.currentThread().getName());
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}
如上面的代码所示,定义了一个初始容量为2,最大容量为100,队列长度为10的线程池,期待的运行结果为:
Thread name=pool-1-thread-1
Thread name=pool-1-thread-2
Thread name=pool-1-thread-3
Thread name=pool-1-thread-4
Thread name=pool-1-thread-5
Thread name=pool-1-thread-6
Thread name=pool-1-thread-7
Thread name=pool-1-thread-8
Thread name=pool-1-thread-9
Thread name=pool-1-thread-10
Thread name=pool-1-thread-3
Thread name=pool-1-thread-5
Thread name=pool-1-thread-2
Thread name=pool-1-thread-1
Thread name=pool-1-thread-4
Thread name=pool-1-thread-10
Thread name=pool-1-thread-7
Thread name=pool-1-thread-6
Thread name=pool-1-thread-9
Thread name=pool-1-thread-8
Thread name=pool-1-thread-3
Thread name=pool-1-thread-4
Thread name=pool-1-thread-1
Thread name=pool-1-thread-5
Thread name=pool-1-thread-2
Thread name=pool-1-thread-8
Thread name=pool-1-thread-6
Thread name=pool-1-thread-7
Thread name=pool-1-thread-9
Thread name=pool-1-thread-10
期待十个线程都可以运行,但实际的执行效果如下:
Thread name=pool-1-thread-1
Thread name=pool-1-thread-2
Thread name=pool-1-thread-2
Thread name=pool-1-thread-1
Thread name=pool-1-thread-1
Thread name=pool-1-thread-2
Thread name=pool-1-thread-1
Thread name=pool-1-thread-2
Thread name=pool-1-thread-2
Thread name=pool-1-thread-1
Thread name=pool-1-thread-2
Thread name=pool-1-thread-1
Thread name=pool-1-thread-2
Thread name=pool-1-thread-1
对比可以看出,用上面的方式定义线程池,最终只有两个线程可以运行,即线程池的初始容量大小。其余线程都被阻塞到了队列ArrayBlockingQueue<>(10)
问题分析
我们知道,Executors框架提供了几种常见的线程池分别为:
- newCachedThreadPool创建一个可缓存线程池,如果线程池长度超过处理需要,可灵活回收空闲线程,若无可回收,则新建线程。
- newFixedThreadPool 创建一个定长线程池,可控制线程最大并发数,超出的线程会在队列中等待。
- newScheduledThreadPool 创建一个定长线程池,支持定时及周期性任务执行。
- newSingleThreadExecutor 创建一个单线程化的线程池,它只会用唯一的工作线程来执行任务,保证所有任务按照指定顺序(FIFO, LIFO, 优先级)执行。
如果将代码中自定义的线程池改为 :
private static ExecutorService es = Executors.newCachedThreadPool();
运行发现,提交的十个线程都可以运行
Executors.newCachedThreadPool()的源码如下:
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
通过对比发现,newCachedThreadPool使用的是 SynchronousQueue<>()而我们使用的是ArrayBlockingQueue<>(10) 因此可以很容易的发现问题出在队列上。
问题解决
将ArrayBlockingQueue改为SynchronousQueue 问题解决,代码如下:
public class ExecutorTest {
private static ExecutorService es = new ThreadPoolExecutor(2,
100, 1000, TimeUnit.MILLISECONDS
, new SynchronousQueue<>());
private static ExecutorService es2 = Executors.newCachedThreadPool();
public static void main(String[] args) {
for (int i = 0; i < 10; i++) {
es.submit(new MyThread());
}
}
static class MyThread implements Runnable {
@Override
public void run() {
for (; ; ) {
System.out.println("Thread name=" + Thread.currentThread().getName());
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}
总结
两个队列的UML关系图
从图上我们可以看到,两个队列都继承了AbstractQueue实现了BlockingQueue接口,因此功能应该相似
SynchronousQueue的定义
* <p>Synchronous queues are similar to rendezvous channels used in
* CSP and Ada. They are well suited for handoff designs, in which an
* object running in one thread must sync up with an object running
* in another thread in order to hand it some infORMation, event, or
* task.
SynchronousQueue类似于一个传递通道,只是通过他传递某个元素,并没有任何容量,只有当第一个元素被取走,才能在给队列添加元素。
ArrayBlockingQueue的定义
* A bounded {@linkplain BlockingQueue blocking queue} backed by an
* array. This queue orders elements FIFO (first-in-first-out). The
* <em>head</em> of the queue is that element that has been on the
* queue the longest time. The <em>tail</em> of the queue is that
* element that has been on the queue the shortest time. New elements
* are inserted at the tail of the queue, and the queue retrieval
* operations obtain elements at the head of the queue.
ArrayBlockingQueue从定义来看就是一个普通的队列,先入先出,当队列为空时,获取数据的线程会被阻塞,当队列满时,添加队列的线程会被阻塞,直到队列可用。
分析
从上面队列的定义中可以看出,导致线程池没有按照预期运行的原因不是因为队列的问题,应该是关于线程池在提交任务时,从队列取数据的方式不同导致的。
jdk源码中关于线程池队列的说明
* <dt>Queuing</dt>
*
* <dd>Any {@link BlockingQueue} may be used to transfer and hold
* submitted tasks. The use of this queue interacts with pool sizing:
*
* <ul>
*
* <li> If fewer than corePoolSize threads are running, the Executor
* always prefers adding a new thread
* rather than queuing.</li>
*
* <li> If corePoolSize or more threads are running, the Executor
* always prefers queuing a request rather than adding a new
* thread.</li>
*
* <li> If a request cannot be queued, a new thread is created unless
* this would exceed maximumPoolSize, in which case, the task will be
* rejected.</li>
从说明中可以看到,如果正在运行的线程数必初始容量corePoolSize小,那么Executor会从创建一个新线程去执行任务,如果正在执行的线程数必corePoolSize大,那么Executor会将新提交的任务放到阻塞队列,除非当队列的个数超过了队列的最大长度maxmiumPooSize。
从源码中找到关于提交任务的方法:
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
从源码中看到 subimit实际上是调用了execute方法
execute方法的源码:
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}
源码中可以看出,提交任务时,首先会判断正在执行的线程数是否小于corePoolSize,如果条件成立那么会直接创建线程并执行任务。如果条件不成立,且队列没有满,那么将任务放到队列,如果条件不成立但是队列满了,那么同样也新创建线程并执行任务。
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